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Geothermal Energy

Science – Society – Technology

Fig. 6 | Geothermal Energy

Fig. 6

From: Stress sensitivity of porosity and permeability under varying hydrostatic stress conditions for different carbonate rock types of the geothermal Malm reservoir in Southern Germany

Fig. 6

Geomechanical characterization of the tested sample material. a Dynamic velocity measurements of the compression wave \({v}_{\mathrm{p}}\) (black) and shear wave \({v}_{\mathrm{p}}\) (white) show a distinct decrease in velocity with increasing initial porosity \({\varPhi }^{i}\) (WTA = Wyllie Time Average). b Rock strength \({\sigma }_{s}\) is negatively correlated with the initial porosity \({\varPhi }^{i}\) and implies a decrease in rock strength with increasing porosity. Several authors published a similar dependency of both parameters based on empirical data. c Wave velocities show an exactly inverse correlation with rock strength, again with a direct relationship through porosity. The dynamic bulk modulus \({K}_{\mathrm{dyn}}\) (d) and the dynamic Young’s modulus \({E}_{\mathrm{dyn}}\) (e) are both positively correlated with the rock strength, indicating increasing stiffness and elasticity of the rock with increasing rock strength. f Results for the dynamic (\({E}_{\mathrm{dyn}}\)) and static (\(E\)) Young’s modulus show a general similarity. However, the measured values for samples with abundant vugs show significantly higher values for \({E}_{\mathrm{dyn}}\), as most of the separated vugs cannot be detected by ultrasonic measurements

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